FIG. 20A shows a structure of a conventional cathode ray tube 1900. As shown in FIG. 20A, the cathode ray tube 1900 includes a panel 1903 having a generally quadrangular container-shape. The panel 1903 includes a panel main body 1901 having an inner curved surface and a side wall 1902 provided around four sides of the panel main body 1901. The cathode ray tube 1900 further includes a funnel 1904 joined to the side wall 1902.
On an inner surface of the panel 1903, a phosphor screen 1905 including a three color phosphor layer of RGB color elements (red, green and blue) is provided. A mask frame 1909 is provided to face the phosphor screen 1905. The mask frame 1909 includes a generally quadrangular frame 1908 and a mask (shadow mask) 1907 attached to the frame 1908 so as to extend over the frame 1908. The mask 1907 is generally quadrangular and has a plurality of electron beam transmission holes or slits 1906.
The funnel 1904 includes a neck 1910, which accommodates an electron gun 1912 for emitting three electron beams 1911. A color image is displayed as follows. The three electron beams 1911 emitted by the electron gun 1912 are deflected by a magnetic field generated by a deflection device 1913 provided on an outer surface of the funnel 1904, and the phosphor screen 1905 is horizontally and vertically scanned with the three electron beams 1911 through the mask frame 1909. In FIG. 20A, reference numeral 1906 represents an inner magnetic shield attached to the frame 1908. In order to display an accurate color image on the phosphor screen 1905 in the cathode ray tube 1900 having the above-described structure, the mask frame 1909 needs to be kept aligned at a prescribed relationship with respect to the three color phosphor layer included in the phosphor screen 1905.
FIG. 20B is a cross-sectional view of the cathode ray tube 1900 shown in FIG. 20A taken along line A—A in FIG. 1. As shown in FIG. 20B, a known system for supporting the mask frame 1909 includes generally V-shaped elastic supports 1914 respectively attached to four corners of the frame 1908 and stud pins 1915 respectively provided on the four corners of the side wall 1902 of the panel 1903. The mask frame 1909 is detachably supported to the panel 1903 by engaging the elastic supports 1914 with the stud pins 1915 respectively.
FIG. 21A shows a structure of another conventional cathode ray tube 2000. FIG. 21B is across-sectional view of the cathode ray tube 2000 shown in FIG. 20A taken along line B—B in FIG. 21A. Identical elements previously discussed with respect to FIGS. 20A and 20B bear identical reference numerals and the descriptions thereof will be omitted. As shown in FIG. 21B, another known system for supporting the mask frame 1909 includes strip-like elastic supports 2001 respectively attached to centers of four sides of the frame 1908 and stud pins 1915 respectively provided at centers of four inner faces of the side wall 1902 of the panel 1903. The mask frame 1909 is detachably supported to the panel 1903 by engaging the elastic supports 2001 with the stud pins 1915 respectively. This system is generally known.
In general, in order to display a color image with no degradation of color purity on a phosphor screen of a cathode ray tube, it is necessary that the three electron beams 1911 transmitted through the holes 1906 formed in the mask 1907 (shadow mask) of the mask frame 1909 should accurately land on respective color areas of the three color phosphor elements of the phosphor screen 1905. In order to realize this state, the positional relationship between the panel 1903 and the mask frame 1909 needs to be kept at a prescribed relationship. Particularly, the distance (q value) between the inner surface of the panel main body 1901 on which the phosphor screen 1905 is provided and a surface of the mask 1907 facing the panel main body 1901 (shadow mask surface) needs to be kept within a prescribed tolerance.
In the state where a cathode ray tube (cathode ray tube 2000, for example) is incorporated in a set such as a TV or an image monitor, vibration from a speaker built into the set or vibration from outside the set is transferred to the cathode ray tube through a cabinet of the set, which undesirably causes the frame 1908 and the mask 1907 to also vibrate through resonance. When the vibration amplitude of the frame 1908 and the mask 1907 exceeds the prescribed tolerance of the q value, the three electron beams 1911 land on offset positions and degradation of color purity occurs. As a result, the image quality is deteriorated.
In addition, when the frame 1908 and the mask 1907 vibrate in the planar direction thereof to the extent that one of the transmission holes 1906 of the mask 1907 reaches an adjacent transmission hole 1906, the three electron beams 1911 do not accurately land on the phosphor layer of the phosphor screen 1905 (mislanding).
In order to restrict the vibration of the mask 1907 and the frame 1908 caused by the vibration transferred from outside the cathode ray tube (cathode ray tube 2000, for example), the vibration of the frame 1908 first needs to be quickly stopped. The reason for this is that unless the frame 1908 stops vibrating, the mask 1907 fixed to the frame 1908 does not stop vibrating. The inside of the cathode ray tube 2000 is in vacuum and the vibration is not attenuated by friction with air. Therefore, the mask 1907 and the frame 1908 are likely to keep vibrating for an extended period of time inside the cathode ray tube 2000. Accordingly, in order to restrict the vibration, it is necessary to provide a structure to cause friction by the vibration inside the cathode ray tube 2000 so that vibration energy is converted into friction energy.
FIGS. 22A, 22B and 22C show a specific structure of the elastic support 1914 shown in FIGS. 20A and 20B, which is disclosed in Japanese Laid-Open Publication No. 9-293459. The elastic support 1914 includes a fixing portion 2101 to be fixed to the frame 1908, an engagement portion 2102 having an engaging hole 2104 for engagement with the stud pin 1915 (FIGS. 20A and 20B), and connection portions 2103a and 2103b for connecting the fixing portion 2101 and the engagement portion 2102. The connection portions 2103a and 2103b are fixed to each other at a welding point 2107 by welding. The elastic support 1914 further includes plate-like bent portions 2105 perpendicularly raised from the fixing portion 2101 and slits 2106 made in the engagement portion 2102 for receiving the bent portions 2105. When the stud pin 1915 is engaged with the engaging hole 2104, the bent portions 2105 are inserted through the slits 2106. Due to such a structure, when the frame 1908 vibrates, the bent portions 2105 cause friction with inner surfaces of the slits 2106, and thus vibration energy is converted to friction energy. Thus, the vibration of the frame 1908 is restricted.
FIGS. 23A and 23B show a specific structure of the elastic support 2001 shown in FIGS. 21A and 21B, which is disclosed in Japanese Laid-Open Publication No. 9-35653. The elastic support 2001 includes a plurality of leaf springs having an identical shape (two leaf springs 2002 and 2003 in the example shown in FIGS. 23A and 23B), and is provided in the vicinity of a center of each of the four sides of the frame 1908 as shown in FIG. 21B. When the frame 1908 vibrates due to vibration externally applied, the elastic support 2001 is deformed and thus friction is caused between the leaf springs 2002 and 2003. Accordingly, the vibration of the frame 1908 is rapidly attenuated. FIG. 23B shows how the elastic support 2001 is engaged with the stud pin 1915.
The elastic support 1914 shown in FIGS. 22A, 22B and 22C has the following problem.
When the frame 1908 vibrates, both of two surfaces of the bent portions 2105 cause friction with inner surfaces of the slits 2106. Accordingly, the vibration is rapidly attenuated. However, when two surfaces perpendicular to each other (surfaces of the bent portions 2105 and the inner surfaces of the slits 2106) rub against each other, the sliding surfaces need to be highly smooth. Otherwise, the surfaces are easily locked by each other. Especially the inner space of the cathode ray tube, which is in high vacuum, has a friction coefficient larger than that in the outside air. Accordingly, there is a high possibility that the surfaces of the bent portions 2105 and the inner surfaces of the slits 2106 are locked by each other so as to be unmovable. Once the bent portions 2105 and the slits 2106 become unmovable, the elastic support 1914 cannot provide its original function. That is, when the electron beams 1911 (FIGS. 20A and 20B) hit the mask 1907 to raise the temperature of the mask 1907 and the mask 1907 expands while the cathode ray tube 1900 is in operation, the elastic support 1914 cannot adjust the position of the frame 1908 so as to correct the positional relationship (q value) between the inner surface of the panel main body 1901 on which the phosphor screen 1905 is provided and the surface of the mask 1907 facing the panel main body 1901 (shadow mask surface). The reason for this is that the elastic support 1914 has a mechanism of correcting the position of the frame 1908 by the elasticity of the engagement portion 2102 and the connecting portion 2103a. 
The elastic support 2001 shown in FIGS. 23A and 23B has the following problems.
The leaf springs 2002 and 2003 having an identical shape are completely superimposed with each other. Accordingly, friction does not occur between the leaf springs 2002 and 2003 unless a force sufficiently large to deform the leaf spring 2002 is applied by the vibration. In addition, since a contact area of the leaf springs 2002 and 2003 is relatively large, the friction coefficient between the leaf springs 2002 and 2003 is large. For this reason also, the vibration of the frame 1908 cannot be restricted by the friction unless the vibration has a relatively large amplitude. Especially against the vibration in the axial direction of the cathode ray tube, the restriction effect of the elastic support 2001 is small due to a very small friction between the leaf springs 2002 and 2003. The friction is very small because the leaf springs 2002 and 2003 are likely to move in the same manner in a superimposed state due to a large friction coefficient. In actuality, when the vibration amplitude of the frame 1908 exceeds, for example, about 100 μm in the axial direction, the degradation of color purity becomes conspicuous. The leaf spring 2002 does not receive a sufficiently large force in response to such a small vibration amplitude, and thus is not deformed much. For these reasons, the elastic support 2001 does not provide a sufficient effect of restricting the vibration and is not practical for use.
The present invention has an objective of providing a cathode ray tube for rapidly attenuating vibration of a frame against vibration transferred from outside and thus preventing degradation of color purity in a color image from occurring due to mislanding of electron beams, and an image display apparatus using such a cathode ray tube.